FSQ0765RS Datasheet, PDF (16/19 Pages) Fairchild Semiconductor – Green-Mode Fairchild Power Switch (FPS) for Quasi-Resonant Operation - Low EMI and High Efficiency

t_smax=21μs
I_DS
t_B=15μs
t_s
I_DS
t_B=15μs
t_s
I_DS
A
I_DS
B
I_DS
t_B=15μs
t_s
I_DS
C
I_DS
I_DS
t_B=15μs
D
t_W=6μs
t_smax=21μs
FSQ0765R Rev. 00
Figure 42. QRC Operation with Limited Frequency
8. AVS (Alternating Valley Switching): Due to the
quasi-resonant operation with limited frequency, the
switching frequency varies depending on input voltage,
load transition, and so on. At high input voltage, the
switching on time is relatively small compared to low
input voltage. The input voltage variance is small and the
switching-frequency modulation width becomes small. To
improve the EMI performance, AVS is enabled when
input voltage is high and the switching on time is small.
Internally, quasi-resonant operation is divided into two
categories; one is first valley switching and the other is
second-valley switching after blanking time. In AVS, two
successive occurrences of first-valley switching and the
other two successive occurrences of second-valley
switching is alternatively selected to maximize frequency
modulation. As depicted in Figure 34, the switching
frequency hops when the input voltage is high. The
internal timing diagram of AVS is described in Figure 35.
f_s
67kHz
59kHz
53kHz
48kHz
Assume the resonant period is 2μs
AVS trigger point
Constant
frequency
CCM
Variable frequency within limited range
DCM
AVS region
1
15μs
1
17μs
1
19 μs
1
21μs
D
C
B
FSQ0765R Rev.00
A
Vin
Figure 43. Switching Frequency Range
V_gate
GateX2
V_gate continued 2 pulses
1st valley switching
V_gate continued another 2 pulses
2nd valley switching
V_gate continued 2 pulses
1st valley switching
One-shot fixed
fixed
fixed
AVS ^triggering _{1st or 2nd is depend on GateX2} de-triggering _triggering
fixed
fixed
fixed
1st or 2nd is dependent on GateX2
V_DS
t_B
t_B
t_B
t_B
t_B
t_B
GateX2: Counting V_gate every 2 pulses independent on other signals.
FSQ0765R Rev. 00
1st valley- 2nd valley frequency modulation.
Modulation frequency is approximately 17kHz.
Figure 44. Alternating Valley Switching (AVS)
© 2008 Fairchild Semiconductor Corporation
FSQ0765RS • Rev. 1.0.2
16
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